A portion of the disclosure of this patent document contains material which is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent or the patent disclosure, as it appears in the Patent and Trademark Office, patent file or records, but otherwise reserves all copyright rights whatsoever.
The present invention relates to methods for computing the x, y and xcex8 orientation of an object in a system, such as orienting an electronic component for placement on a PC board, as in a pick and place machine.
Electronic shadowing techniques incorporated in optical-based sensors are also in widespread use today in the electronics industry in pick and place machines for mounting electronic components on printed circuit boards. One of the most commonly used position alignment sensors is manufactured by CyberOptics Corporation in Golden valley, Minn. and is sold as a LaserAlign(copyright) component alignment sensor. LaserAlign type sensors use a light focused into a stripe of light, which is typically incident on the side of an electronic component, thereby forming a shadow which is cast onto a detector. When the electronic component is rotated (by a nozzle controlled in x, y and z direction by the pick and place machine), the shadow cast on the detector changes in width. When the width of the shadow is minimized at a cusp, the position of the component in one axis and its orientation (i.e., angular position) of the component with respect to the nozzle axis of rotation is known. The pick and place machine then rotates the component approximately ninety degrees, and completes the same process again to establish the position of the component in another direction. The output of the sensor and the attendant processing of the sensor output provide an indication of translational and rotational movements which the pick and place machine must perform before the component can be accurately set onto the PC board U.S. Pat. No. 5,278,634 to Skunes et al., owned by the same assignee as the present invention and hereby incorporated by reference, discloses a LaserAlign(copyright) sensor.
The orientation process is generally carried out while the pick and place machine is transporting the component to a target printed circuit for placement. When the orientation process is carried out simultaneously with the transport of the component, the orientation process is sometimes referred to as an xe2x80x9con-headxe2x80x9d or an xe2x80x9con-the-flyxe2x80x9d measurement. Conversely, xe2x80x9coff-headxe2x80x9d measurements are made when the sensor is not affixed to the pick and place head but rather, is stationary relative to the head.
However, data from these sensors is sometimes less than optimal for orienting the component when the component does not have sharp corners. Further, it is sometimes desirable to have more resolution of the outline of a complex component without sacrificing speed.
One aspect of the present invention includes a novel method of providing a set of vertices representative of the outline of an opaque object so that an outline of the object can be made from the set of vertices. The method includes the step of casting a shadow of the object onto a detector while the object is held at an angle and then detecting at least one edge of the shadow image. Once at least one of the edges in the shadow image is known, a data pair formed by the at least one shadow edge and its corresponding angle is preferably stored. The steps of casting and detecting are repeated until a plurality of data pairs is stored. Finally, a set of vertices is computed from the plurality of data pairs. An outline of the object can be made by plotting the set of vertices, for example, for use in viewing an outline of the object.
The method of the present invention is useful in the electronic assembly industry, where pick and place machines accurately place electronic components on printed circuit boards. Once the set of vertices of the component is known from the present invention, the position and angle of the component relative to a known reference axis in the pick and place machine can be computed and compared against the desired position and angle of the component on the printed circuit board. A correction instruction for the pick and place machine is computed as a function of the desired position and the present position, which the pick and place machine executes so as to accurately place the component onto the printed circuit board.
In another aspect, an optical sensor with a light source and a detector is also disclosed which practices the shadow casting step of the present invention. The sensor is coupled to processing electronics of the present invention which locate the edge of the shadow image received from the detector in the sensor, compute the set of vertices and optionally reconstruct the outline of the component under test. In an alternate embodiment, the edge detection and processing electronics are located in the sensor.